Nil

In Ruby, nil is a special value. It indicates a value that is not present. Methods often return nil when they attempt to locate an element that does not exist.

We must check for nil in many locations in our code. Uninitialized fields in classes are nil too. Nil is everywhere: without nothing, there cannot be something.

First example

This example shows how nil is used with the Array class. We create an array with 3 integer elements. We then call the index method on the array.

# An array of three elements.
values = Array[10, 30, 50]

# Try to find an element that does not exist.
if values.index(70) == nil
    # This is reached.
    puts "Result is nil"
end
Result is nil

NoMethodError

We cannot call methods on the nil object. This will cause a NoMethodError. And at this point, program execution ceases. We must first test references to see if they are nil.

# An example hash.
hash = Hash["a" => "dog"]

# Get an element that does not exist.
v = hash["b"]

# We cannot use length on it.
puts v.length()
...undefined method 'length' for nil:NilClass (NoMethodError)

Uninitialized fields

A field in a class is automatically set to nil before it is assigned a value. So we can check for nil to see if a field has been initialized yet.

class Box
    def size
        @value
    end
end

# Create a new class instance.
x = Box.new

# A field on a class is nil until initialized.
if x.size == nil
    puts "NIL"
end
NIL

Return nil

Methods return nil if no other return value is specified. Often, nil is returned when no value can be found. Here, analyze() returns nil unless "n" equals 2.

def analyze(n)
    # This method returns nil unless n equals 2.
    if n == 2
        return n * 2
    end
end

puts "Result is nil" if analyze(3) == nil
Result is nil

A review

Any class in Ruby can be nil. If our program logic can possibly result in a nil object, we must first test for nil to prevent errors.